In a semiconductor device such as a diode, a BJT, a superjunction MOSFET, or an IGBT in which a number of semiconductor devices are formed in a semiconductor wafer, a planar type junction termination structure is mainly used as a breakdown voltage blocking structure of the device. In the planar type junction termination structure, the termination of a p-n junction is made to intersect the principal surface of a semiconductor wafer and the surface of the intersection is coated with a passivation film such as an insulator film for protection.
A planar type breakdown voltage blocking structure is normally arranged around the outer periphery of an active region positioned at the central section of a semiconductor device. The breakdown voltage blocking structure has a junction termination structure including a curved p-n junction face linking to the outer periphery of a planar p-n junction face corresponding to an active region. In the planar type breakdown voltage blocking structure having the curved p-n junction face, a reverse voltage applied to a p-n main junction is liable to form a section with high electric field strength due to an electric field concentration appearing at a region beneath the curved junction face in comparison with a region beneath the planar junction face in the active region. This, when no measure is taken, causes the electric field strength at the region beneath the curved junction face to reach the critical electric field strength of breakdown determined by the kind of a semiconductor crystal by an applied voltage lower than the design breakdown voltage at the planar junction. Thus, the device has a low breakdown voltage.
As measures against this, an electric field concentration at a region beneath a curved junction face has been previously reduced to enhance the breakdown voltage of a device by providing the following breakdown voltage blocking structures. The breakdown voltage blocking structures are those such as a breakdown voltage blocking structure 121 including a floating guard ring structure 131 and a field plate structure 132 as shown in FIG. 21, which illustrates a cross-sectional view of a planar type junction termination structure around the outermost periphery of a planar type semiconductor substrate of a semiconductor device, and a RESURF structure (not illustrate) that will be described later (see JP-A-6334188), or some combination of them. In general, the surface of the breakdown voltage blocking structure 121 of the planar type semiconductor substrate is often formed flush with the surface of an active region 110 around the outer periphery of the active region 110. Furthermore, the necessary width for the breakdown voltage blocking structure on the surface of the semiconductor substrate increases with an increase in the breakdown voltage. Therefore, particularly in a high breakdown voltage semiconductor substrate (chip), the area ratio of the breakdown voltage blocking structure to the entire area of the chip becomes relatively larger as the area of the chip becomes smaller, which makes the chip disadvantageous at to electrical characteristics and cost.
FIG. 22 is a cross-sectional view showing a breakdown voltage blocking structure (junction termination structure) around the outermost periphery of a related trench type semiconductor device. The semiconductor device has a semiconductor wafer 150 in which a p-n main junction 117 is formed by providing a p− epitaxial layer 112 on an n++ semiconductor substrate 111. In the semiconductor wafer 150, an inclined trench 115 is formed from the principal surface of the semiconductor wafer 150 with a depth deeper than the position of the main p-n junction 117 by a trench forming technology with dry or wet etching. On the inner surface of the inclined trench 115, a trench inner surface layer 114 of an n+-type is formed. The trench inner surface layer 114 is formed in contact with the p− drift layer 112 (the p− epitaxial layer 112) with one surface with an impurity dose ten times or more the dose for the p− drift layer 112. As a result, the trench inner surface layer 114 is to connect the n++ semiconductor substrate 111 and an n+-layer 113, formed on the respective principal surfaces of the p− drift layer 112, with the same conductivity type. A reverse bias voltage applied to the main p-n junction 117 makes an end of a depletion layer formed on the side of the p− drift layer 112 extend to the top principal surface of the p− drift layer 112. The top principal surface of the p− drift layer 112 is protected by a protection film (not shown) such as an oxide film, by which a breakdown voltage is to be held. For obtaining a high breakdown voltage also in such a breakdown voltage blocking structure, a width (distance) 116 on the top surface must be increased. The increased width 116 reduces an effective active region area by the amount of increase. This, as described before, is more disadvantageous for a smaller area chip in current capacity and as well as in cost.
Such a planar type breakdown voltage blocking structure has been already well known, where a trench is applied to extension of an end of a depletion layer from a drift layer to the top principal surface of a substrate, for instance in JP-A-2-22869, which corresponds to U.S. Pat. No. 4,904,609. Descriptions of similar structures are found in other patent documents, including for example FIG. 15(c) and FIG. 24 of JP-A-2001-185727. None of the described structures, however, provides improvements relating to the area ratio of the surface of a breakdown voltage blocking structure to the entire area of a chip.
Referring FIG. 23, which illustrates a reprint of the breakdown voltage blocking structure around the outermost periphery of the trench type semiconductor device (a diode) described in JP-A-2005-136064, a related technology of improving the area ratio is presented. The device is a non-planar type semiconductor device provided with a junction termination on the inner surface of a trench 605, which device is a kind of a mesa semiconductor transistor. The junction termination is inclined at an angle in the negative bevel direction to a p-n main junction 600 between a P+ anode region 604 and an N− drift layer 601. In the device, when a reverse bias voltage (with a cathode electrode 609 made positive and an anode electrode 608 made negative) is applied to the p-n main junction 600, the end of a depletion layer spreads only to the trench 605 without extending to the principal surface of a semiconductor wafer. This way, a kind of a RESURF structure is formed in which a P− surface region 606 extends from the P+ anode region 604 along the inner surface of the trench 605. By the thus formed RESURF structure, extension of the depletion layer in the P− surface region 606 is controlled to reduce the electric field concentration in the inner surface of the trench 605 for inhibiting reduction in the breakdown voltage, as well as improving the area ratio of the breakdown voltage blocking structure to the entire chip area.
However, when the breakdown voltage blocking structure shown in FIG. 23 is applied to a device having a drift layer with a relatively high resistivity such as a superjunction MOSFET, an IGBT, or a diode (in particular a superjunction structure close to a charge balanced state with the average impurity concentration close to an intrinsic one), electric field strength tends to be high at the interface between the N− drift layer 601 and an N++-layer 602. As a result, the semiconductor device described in JP-A-2005-136064 (FIG. 23) sometimes causes breakdown at a low applied voltage, so that it is hard to ensure a sufficiently high breakdown voltage.
This will be explained in the following. FIG. 24 is a diagram showing electric field strength distributions along line B-B′ of FIG. 23 when a reverse bias voltage is applied to the device shown in FIG. 23. In the diagram, a distance from the principal surface of the device is taken on the horizontal axis and an electric field strength is taken on the vertical axis. Moreover, the notation “n++” in the diagram represents the N++-layer 602 in FIG. 23 and the notation “n−” in the diagram represents the N− drift layer 601. The notation “SJ” represents the superjunction structure. The dotted line with the indication “TYPE-1” represents the electric field strength distribution from the N− drift layer 601 or the superjunction structure (hereinafter also referred to as the “SJ”) to the N++-layer 602 when the N− drift layer 601 has a low resistivity. The solid line with the indication “TYPE-2” represents the electric field strength distribution when the N− drift layer 601 has a high resistivity. In the diagram, it is known that, in the N− drift layer 601 with any resistivity, at an interface shown with an arrow, i.e., at a surface of the interface between the N− drift layer 601 and the N++-layer 602 (on the surface in the trench 605), the electric field strength becomes so high as to reach a value approximately equal to that of the critical electric field strength of the semiconductor crystal (shown by “E(CRIT.) on the vertical axis). In this way, with the structure shown in FIG. 23, a sufficient design breakdown voltage sometimes could not be ensured.
There still remains a need for providing a semiconductor device that prevents the electric field strength from reaching the critical electric field strength, which can break down the breakdown blocking structure (hereinafter referred to as a “junction termination structure”), prior to in an active region, as well as improving the area ratio of the junction termination structure to the entire chip area. The present invention addresses this need.